High-power and high-frequency heretostructure field-effect transistor

ABSTRACT

In an HEMT device, a gate region is formed in a wafer having a channel layer, a barrier layer, and a passivation layer, overlying each other. Drain and source electrodes are formed in the wafer, on different sides of the gate region. A dielectric layer is formed over the gate region and over the passivation layer. Selective portions of the dielectric layer are removed by a plurality of etches so as to form one or more cavities between the gate region and the drain electrode. The one or more cavities have a plurality of steps at an increasing distance from the wafer moving from the gate region to the drain electrode. The cavity is then filled with conductive material to form a field plate coupled to the source electrode, extending over the gate region, and having a surface facing the wafer and having a plurality of steps.

BACKGROUND Technical Field

The present disclosure relates to a high-power and high-frequencyheterostructure field-effect transistor.

Description of the Related Art

As is known, AlGaN/GaN heterostructures have drawn high interest formanufacturing high-power and high-frequency field-effect transistors, byvirtue of the high breakdown voltage of the material and the highdensity and mobility of carrier ions in the two-dimensional electron gas(2DEG).

High-electron-mobility transistors (HEMTs) based on AlGaN/GaN aregenerally grown on a silicon substrate, since cheap substrates based ofgallium nitride (GaN) are not available. Therefore, the device ishorizontal and the source, drain, and gate electrodes are arranged onthe upper surface of the wafer. Thus, the wafer breakdown voltage islinked both to the vertical breakdown voltage and to the lateraldistance between the source and the drain electrodes.

Although gallium nitride and its alloys are wide band-gap and highbreakdown semiconductors, one of the critical aspects of HEMT powerdevices of gallium nitride is obtaining a good electrical fielddistribution during switch-off.

Therefore, the structure of an HEMT is carefully studied in order tooptimize the electrical field distribution and to allow the device tooperate at a maximum switching rate, when subjected to a high voltagevariation. Without an appropriate shaping of the existing electricalfield, in fact, the latter may cause injection of electrons in traps,thus degrading the output current and the on-state resistance duringswitching. This phenomenon is frequently referred to as dynamicdegradation, current drop, or DC-RF dispersion.

To prevent this phenomenon, in microwave applications, it has beenproposed to use a V-shaped gate region with an integrated slant fieldplate (see, e.g., Y. Dora, et al. “High Breakdown Voltage Achieved onAlGaN/GaN HEMTs With Integrated Slant Field Plates”, IEEE ELECTRONDEVICE LETTERS, Vol. 27, N. 9, September 2006, pp. 713-715 and R. Chu etal. “V-Gate GaN HEMTs for X-Band Power Applications”, IEEE ELECTRONDEVICE LETTERS, Vol. 29, No. 9, September 2008, pp. 974-976). Forhigh-voltage applications, the use of a structure has also been proposedhaving a plurality of separate, overlying field plates, to increase theoperating voltage (multiple-field-plate solution, see, e.g., H. Xing etal. “High-Breakdown Voltage AlGaN/GaN HEMTs Achieved by Multiple FieldPlates”, IEEE ELECTRON DEVICE LETTERS, Vol. 25, No. 4, April 2004, pp.161-163). Another known solution (see R. Chu et al. “1200-V Normally OffGaN-on-Si Field-Effect Transistors with Low Dynamic On-Resistance”, IEEEELECTRON DEVICE LETTERS, VOL. 32, No. 5, May 2011, pp. 632-634) shows adifferent multiple field plate having overlying portions, in mutualelectrical contact. A similar approach is described in U.S. Pat. No.8,530,978 B1, but with a sloped field plate connected to the sourceelectrode.

In particular, the device described in U.S. Pat. No. 8,530,978 B1comprises a stack of AlGaN/GaN epitaxial layers on a silicon substrate.A passivation layer of dielectric material covers the stack of epitaxiallayers and accommodates source and drain electrodes, a gate electrode, afirst field plate connected to the gate region, and a second field plateconnected to the source electrode. The second field plate has a slopedbottom surface. The first field plate, connected to the gate region, isintended to prevent charge trapping at low voltage.

The sloped shape of the second field plate is obtained by exposing aphotoresist through a greyscale mask. This mask allows to modulate theintensity of the passing light and to obtain a sloped profile in thephotoresist. This profile is then transferred onto the underlyingdielectric layer by dry etching. The process for forming the slopedfield plate is simpler and faster than the process for forming aplurality of separate field plates since the latter requires numeroussuccessive metal deposition and definition steps.

The presence of a sloped field plate enables the increase of the dynamicon-state resistance at 600 V to be reduced from 47%, that may beachieved with the multiple-field-plate device, to 19%. The improvementof performance is thought to be due to a more linear profile of theelectrical field, which can be achieved by virtue of the sloped fieldplate.

However, also the solution with sloped field plate is not free fromdisadvantages. In fact, it involves a complex manufacturing process anda final planarization step. Forming the sloped field plate is thuscostly and may not be sufficiently controllable.

BRIEF SUMMARY

At least some embodiments of the present disclosure provide a differentmanufacturing process and a new HEMT device having a field plate with asloped surface.

According to the present disclosure, a manufacturing process and aheterostructure field-effect transistor are provided.

In practice, to form the field plate, the dielectric layer overlying thegate, drain, and source regions is subjected to a plurality ofsuccessive etching steps such as to form a cavity, connected or notconnected, with a bottom surface or an underside envelope having aplurality of steps, thereby meaning a plurality of discontinuous surfaceportions adjacent to each other, wherein each surface portion extends ata different distance from the underlying wafer with respect to adjacentsurface portions.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present disclosure, preferredembodiments thereof are now described, purely by way of non-limitingexample, with reference to the attached drawings, wherein:

FIGS. 1-11 are cross-sections through a semiconductor material wafer insuccessive manufacturing steps of a first embodiment of the presentfield-effect HEMT device;

FIGS. 12-15 are cross-sections through a semiconductor material wafer insuccessive manufacturing steps of a second embodiment of the presentfield-effect HEMT device; and

FIG. 16 shows a cross-section of a variant of the device of FIG. 11.

DETAILED DESCRIPTION

FIG. 1 shows a portion of a wafer 100 for manufacturing an HEMT device,according to one embodiment of the present device. In particular, thedescribed embodiment regards a heterostructure field-effect transistorof AlGaN/GaN.

The wafer 100 comprises a substrate 101 of, e.g., silicon, siliconcarbide (SiC), or sapphire (Al₂O₃), overlaid by a buffer layer 102, forexample of aluminum gallium nitride (AlGaN) or of indium gallium nitride(InGaN). A channel layer 103 extends on the buffer layer 102 and is hereof intrinsic gallium nitride (GaN), having a thickness of, e.g., ca. 10nm to ca. 10 μm. A barrier layer 104, here of aluminum and galliumnitride (AlGaN) of an intrinsic type, extends on the channel layer 103,in contact therewith, and has a thickness of, e.g., ca. 5 nm and ca. 400nm. A passivation layer 105 of dielectric material such as siliconnitride (Si₃N₄) or silicon oxide (SiO₂) extends on the barrier layer104.

As shown in FIG. 2, the passivation layer 105 and the barrier layer 104are selectively removed at the portion of the wafer 100 where, insubsequent steps, a gate region of the HEMT device is to be formed. Forinstance, lithography steps are carried out, including etching of thepassivation layer 105 and dry etching of the barrier layer 104 so as toremove selective portions thereof. A trench 110 is thus formed, whichextends throughout the thickness of the passivation layer 105 and of thebarrier layer 104.

Next, FIG. 3, a gate insulation layer 111 is deposited, for example madeof a material chosen from aluminum nitride (AlN), silicon nitride (SiN),aluminum oxide (Al₂O₃), or silicon oxide (SiO₂). The gate insulationlayer 111 may have a thickness of between 1 and 50 nm, for example, 20nm.

Then, as shown in FIG. 4, a conductive layer 112 is deposited on thegate insulation layer 111 and fills the trench 110. For instance, theconductive layer 112 is made of metal material, such as tantalum (Ta),tantalum nitride (TaN), titanium nitride (TiN), palladium (Pa), tungsten(W), tungsten silicide (WSi₂), titanium aluminum (Ti/Al), or nickel gold(Ni/Au).

The conductive layer 112 is then selectively removed (FIG. 5), throughlithographic and etching steps per se known so as to remove theconductive layer 112 from the wafer 100, except for the portionextending in the trench 110 and for an adjacent portion, projecting fromthe wafer 100. A gate metallization 112′ is thus formed. The gatemetallization 112′ and the gate insulation layer 111 form, as a whole,the gate region, designated as a whole by 109, of the HEMT device. Then,one or more further masked etching steps of the gate insulation layer111, of the passivation layer 105, and of the barrier layer 104 arecarried out at the areas of the wafer 100 where the source and drainelectrodes of the HEMT device are to be formed. In particular, openings113 and 114 are formed on opposite sides of the gate region 109 and at adistance therefrom.

Next, as shown in FIG. 6, ohmic contacts are formed to provide thesource and drain electrodes 115, 116. To this end, conductive materialis deposited, in particular metal such as titanium (Ti) or aluminum(Al), or their alloys or compounds, by sputtering or evaporation, on thewafer 100 and in particular inside the openings 113, 114. The metallayer thus deposited is then etched so as to remove it from the wafer100, except for the portions extending in the openings 113 and 114,forming therein, respectively, source and drain electrode electrodes115, 116.

Then, FIG. 7, a dielectric layer 120 is deposited, for example ofsilicon nitride or silicon oxide or TEOS(tetraethylorthosilicate)/SOG(Spin-On-Glass)/TEOS, which coats thesource and drain electrodes 115, 116, the gate region 109, and the gateinsulation layer 111. The dielectric layer 120 thus obtained is thenplanarized (for example via CMP—Chemical Mechanical Polishing). Next,portions of the dielectric layer 120 are selectively removed, by aphotolithographic process followed by an etch, for example a dry plasmaetch, a wet etch or a combination of these, at the source and drainelectrodes 115, 116.

Next, FIG. 8, the dielectric layer 120 is etched, by a lithographicprocess followed by etching, for example dry plasma etching, wetetching, or a combination of these, to form a first trench 121. Thefirst trench 121 extends directly over the area of the wafer 100 whichis arranged between the gate region 109 and the drain electrode 116,through most of the thickness of the dielectric layer 120, but stops ata distance from the gate insulation layer 111, for example at 50 nm.

Next, FIG. 9, a second trench 122 is created, with a same process,adjacent to the first trench 121. In particular, the second trench 122is arranged between the first trench 121 and the vertical line passingthrough the drain electrode 116 and extends through the dielectric layer120; moreover, its depth is slightly smaller than the first trench 121,for example its bottom end is arranged at 200 nm from the gateinsulation layer 111.

Then, FIG. 10, using the same process as described with reference toFIGS. 8 and 9, a plurality of trenches 123 are formed, adjacent to thesecond trench 122. The plurality of trenches 123 is arranged between thesecond trench 122 and the vertical line passing through the drainelectrode 116 and extends through the thickness of the dielectric layer120 so that each trench 123 is less deep than the adjacent one, movingtoward the gate region 109, and deeper than the adjacent one, movingtoward the drain electrode 116. As a whole, the plurality of trenches121-123 forms a non-connected cavity 126.

Next, FIG. 11, a metal layer is deposited on the dielectric layer 120.The metal layer is in direct electrical contact with the sourceelectrode 115 and drain electrode 116 where the dielectric layer 120 hasbeen removed. Then, the metal layer is defined, according to aphotolithographic process, thereby forming a field plate 131 and a draincontact 134. The field plate 131 comprises a planar portion 125 on thedielectric layer 120 and a projecting region formed by a plurality offingers 124 projecting inside trenches 121, 122, and 123. The planarportion 125 is in direct electrical contact with the source electrode115 and forms a source contact. The drain contact 134 is in directelectrical contact with the drain electrode 116.

The field plate 131 thus has, towards the wafer 100, an envelope surfaceor profile 133 (see in particular the enlarged detail) defined by aplurality of steps 132 having a distance, from the wafer 100, thatincreases moving from the gate region 109 to the drain electrode 116. Inpractice, the envelope surface 133 defines on average a surface that issloped with respect to the surface plane of the wafer 100 and thusallows an optimization of the electrical field to be obtained andtherefore a high breakdown voltage also during high-power operation, asdiscussed previously.

The final wafer 100′ thus obtained is thus planar and does not requirefurther planarization steps. After dicing, a plurality of normally offHEMT devices 50 is obtained.

In a different embodiment, initially the same manufacturing steps arecarried out as described previously with reference to FIGS. 1-7. Next,FIG. 12, the dielectric layer 120 is etched, according to the sameprocess as the previous embodiment, in a first area 135 arranged betweenthe vertical line passing through the gate region 109 and the verticalline passing through the drain electrode 116, forming a cavity 140.

Here, the dielectric layer 120 is etched for a depth smaller than in theembodiment of FIG. 8, for the reason that will be explained hereinafter.

Then, FIG. 13, a second etch of the dielectric layer 120 is carried out.In detail, the second etch removes selective portions of the dielectriclayer 120 in the first area 135 and in a second area 136, adjacent andcontiguous to the first area 135, moving toward the drain electrode 116.Thereby, a widening and deepening of the cavity 140 is obtained, and thecavity now occupies the second area 136 and, in the first area 135, hasa depth caused by the first and second etches.

Next, FIG. 14, one or more further etches of the dielectric layer 120are performed in the first and second areas 135, 136 and in a pluralityof areas 137, adjacent and contiguous to each other as well as to thesecond area 136, arranged between the second area 136 and the verticalline passing through the drain electrode 116. Thereby, a furtherwidening and deepening of the cavity 140 is obtained. Consequently, thecavity 140, in the first area 135, has a depth given by the sum of allthe etches, in the second area 136 has a depth determined by the sum ofthe second and further etches and, in the plurality of areas 137, adepth that is progressively smaller. In this way, the cavity 140, ineach further area 137, has a depth smaller than in an adjacent areamoving toward the gate region 109 and a depth greater than in anadjacent area 137 moving toward the drain electrode 116.

At the end of the etching phase, in the embodiment of FIGS. 12-15, asingle cavity 140 extends inside the dielectric layer 120; the cavity140 is delimited, towards the wafer 100, by a staircase surface 144, thesteps whereof are arranged at an increasing distance from the wafer 100moving from the gate region 109 to the drain electrode 116.

Next, FIG. 15, a metal layer is deposited over the dielectric layer 120,similar to what described with reference to FIG. 11. Then, the metallayer is defined, according to a photolithographic process, so as toobtain a field plate 142 and the drain contact 134. Here, the fieldplate 142 comprises the planar portion 139 and a single projectingportion 145, the bottom surface whereof (towards the wafer 100)reproduces the shape of the cavity 140 and thus has a plurality of steps143.

After dicing the final wafer 100′, an HEMT device 150 is obtained, whichalso here is of a normally off type.

FIG. 16 shows a different embodiment where the gate region 209 extendsthrough the passivation layer 205 alone and not through the barrierlayer 204, thus forming an HEMT device 250 of a normally on type.

In FIG. 16, the field plate 131 is formed as in the first embodiment andhas a plurality of projecting fingers 124 delimited by an envelopesurface 133 defined by a plurality of steps 132.

As an alternative thereto, an HEMT device of a normally on type may beformed with a field plate having a single projecting portion, similar tothe field plate 142 of FIG. 14, in a manner not shown.

The HEMT device described herein and the corresponding manufacturingprocess are advantageous as compared to the known solutions. In fact,the device has high planarity following upon a single planarizationoperation, thus facilitating electrical connection and not requiringrepeated costly planarizations. Furthermore, the manufacturing processdescribed comprises manufacturing steps that are in per se standard insemiconductor technology and are thus reliable and easy to control. Thefield plate 131, 142, as a whole, has a profile sloped towards the wafer100, thus ensuring the desired optimization of the electrical field andvoltage strength. The final HEMT device thus has high reliability.

Finally, it is clear that modifications and variations may be made tothe process and to the device described and illustrated herein, withoutthereby departing from the scope of the present disclosure. Forinstance, the various embodiments described may be combined so as toprovide further solutions.

In addition, in all the solutions, the number of steps of the bottomsurface of the field plate (defined by the bottom end of the projectingfingers 124 of FIG. 11 or by the steps 132 of the single projectingportion 143 of FIG. 15) may range from two on.

Furthermore, the depth, width, and mutual distance of each step of thefield plate 131, 142 may vary according the design specifications, andthese parameters may be optimized by the designer without anydifficulty.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

The invention claimed is:
 1. A process for manufacturing aheterostructure field-effect transistor, comprising: in a waferincluding a channel layer of a first semiconductor material, a barrierlayer of a second semiconductor material, and a passivation layer ofdielectric material, forming a gate region extending in the wafer;forming drain and source electrodes extending in the wafer on differentsides of the gate region; forming a dielectric layer over the gateregion and the passivation layer; and removing selective portions of thedielectric layer through a plurality of etches so as to form one or morecavities directly above a region of the wafer between the gate regionand the drain electrode, the one or more cavities including a pluralityof steps at different distances from the wafer; and completely fillingthe one or more cavities with conductive material to form a field plateelectrically coupled to the source electrode and extending over the gateregion and inside the one or more cavities, the field plate having asurface that faces the wafer and includes a plurality of steps in therespective steps of the one or more cavities.
 2. The process accordingto claim 1, wherein: forming the one or more cavities comprises forminga plurality of adjacent trenches arranged at a distance from each other,the trenches having different depths and form the steps of the one ormore cavities; and completely filling the one or more cavities includesforming a plurality of projecting fingers, distinct from each other,that completely fill the trenches and extend towards the body.
 3. Theprocess according to claim 1, wherein forming the one or more cavitiescomprises carrying out a first etch of the dielectric layer in a firstarea and carrying out a second etch of the dielectric layer in the firstarea and in a second area contiguous to the first area so as to form asingle cavity that includes the plurality of steps of the one or morecavities.
 4. The process according to claim 3, wherein forming the oneor more cavities comprises carrying out a third etch of the dielectriclayer in the first area, in the second area, and in a third area of thedielectric layer that is contiguous to the second area.
 5. The processaccording to claim 1, wherein forming the one or more cavities comprisesforming steps arranged at an increasing distance from the wafer movingfrom the gate region to the drain electrode.
 6. The process according toclaim 1, wherein the conductive material is metal.
 7. The processaccording to claim 1, comprising defining the conductive material andforming a planar portion and a source contact of the field plate, aswell a drain contact after completely filling the one or more cavities.8. The process according to claim 2, wherein the field plate includes aplanar portion extending over the dielectric layer and the projectingfingers extend from the planar portion inside the dielectric layertowards the body.
 9. A method of forming heterostructure field-effecttransistor, comprising: forming source and drain electrodes extending ina body including a channel layer of a first semiconductor material; abarrier layer of a second semiconductor material, over the channellayer; and a passivation layer, over the barrier layer; forming a gateregion extending in the body between the source and drain electrodes;forming a dielectric layer over the gate region and over the passivationlayer; and forming a field plate of conductive material coupled to thesource electrode and extending inside the dielectric layer in an areacomprised between the gate region and the drain electrode, the fieldplate having a surface facing the wafer and having a plurality of steps,wherein the field plate comprises a plurality of projecting fingers,distinct from each other, extending inside the dielectric layer towardsthe body.
 10. The method according to claim 9, wherein the steps have anincreasing distance from the body moving from the gate region to thedrain electrode.
 11. The method according to claim 9, wherein the fieldplate comprises a planar portion extending over the dielectric layer andthe projecting fingers extend from the planar portion inside thedielectric layer towards the body.
 12. The method according to claim 9,wherein the projecting fingers include a longest projecting finger thatis positioned closed to the gate region among the projecting fingers.13. The method according to claim 9, wherein forming the gate regionincludes: forming a trench in the body; forming a gate insulating layerlining walls of the trench; forming a gate electrode in the trench andseparated from the body by the gate insulation layer, wherein the gateinsulation layer extends between the passivation layer and thedielectric layer.
 14. The method according to claim 9, wherein the gateregion extends inside the passivation layer.
 15. The method according toclaim 9, wherein the gate region extends inside the passivation layerand the barrier region.
 16. The method according to claim 9, wherein thefirst and second semiconductor materials have different band gaps andcomprise elements of Groups III-V of the Periodic Table.
 17. The methodaccording to claim 16, wherein the channel layer is of gallium nitride,and the barrier layer is of aluminum gallium nitride.
 18. The transistoraccording to claim 9, wherein each projecting finger includes first andsecond lateral sides opposite to each other and contacting thedielectric layer.
 19. A method of making field-effect transistor,comprising: forming source and drain electrodes extending in asemiconductor body; forming a gate region extending in the body betweenthe source and drain electrodes; forming a dielectric layer over thegate region and over the semiconductor body; and forming a field plateof conductive material coupled to the source electrode and extendinginside the dielectric layer in an area comprised between the gate regionand the drain electrode, the field plate having a surface facing thewafer and having a plurality of projecting fingers extending into thedielectric layer towards the body and having different lengths, theprojecting fingers including a longest projecting finger that ispositioned closest to the gate region among the plurality of projectingfingers.
 20. The method according to claim 19, further comprisingremoving selective portions of the dielectric layer through a pluralityof etches so as to form one or more cavities directly above a region ofthe wafer between the gate region and the drain electrode, the one ormore cavities including a plurality of steps at different distances fromthe wafer, wherein the forming the field plate includes filling the oneor more cavities with conductive material.
 21. The method according toclaim 20, wherein forming the one or more cavities comprises carryingout a first etch of the dielectric layer in a first area and carryingout a second etch of the dielectric layer in the first area and in asecond area contiguous to the first area so as to form a single cavitythat includes the plurality of steps of the one or more cavities. 22.The method according to claim 20, comprising defining the conductivematerial and forming a planar portion and a source contact of the fieldplate, as well a drain contact after filling the one or more cavitieswith the conductive material.
 23. The method according to claim 19,wherein the field plate includes a planar portion extending over thedielectric layer and the projecting fingers extend from the planarportion inside the dielectric layer towards the body.